1. Technical Field
The present invention relates to a droplet discharging method and a droplet discharging apparatus.
2. Related Art
As an example of a droplet discharging apparatus, an ink jet printer which records (prints) an image by discharging ink droplets on various recording media such as a sheet or film is known. The ink jet printer alternately repeats, for example, a dot forming operation of, while moving (scanning) a head provided with a plurality of nozzles in a main scanning direction, discharging ink from each of the nozzles onto a recording medium and a transporting operation of moving (transporting) the recording medium in a sub-scanning direction perpendicular to the main scanning direction to form dots (dot rows) which are lined up in the main scanning direction to be lined up in the sub-scanning direction, thereby forming an image on the recording medium.
In this type of ink jet printer, as a method for further increasing a recording speed, a method of increasing the number of nozzles has been employed. Specifically, this is a method of increasing the number of dots which are discharged and formed in a single scanning process by increasing the number of nozzles per head or lining up a plurality of heads to increase a recording speed. As a high-speed recording method, for example, there is a method (band recording method) of forming an image by completing image formation of a width (band) corresponding to the length of a row constituted by a plurality of nozzles lined up in a sub-scanning direction through a single scanning process, transporting a recording medium in the sub-scanning direction by the width, forming bands to be lined up in the sub-scanning direction so as to allow the end portion of the formed band to abut the end portion of the band formed by the subsequent scanning process, and repeating the above operations. In the case of the band recording method, there may be a case where striped patterns (banding) are formed along the boundary between the bands. This is caused by variations in accuracy of transport in the sub-scanning direction, an effect of the difference between the characteristics of switching parts of nozzle rows, and the like.
In JP-A-6-47925, as a method of suppressing the degradation in image quality due to the banding, a method of performing partial overlap (POL control) is suggested. POL control is a method of dispersing variations in characteristics or accuracy, and specifically, as the simplest example, is a method of causing the lower end region of a band formed by a scanning process of a head and the upper end region of a band formed by the subsequent scanning process of the head to overlap each other and forming dots of a part of the lower end region of the initially formed band and dots of a part of the upper end region of the subsequently formed band to overlap each other in the same region. In JP-A-10-323978, a method of forming a high-quality image by using a plurality of nozzle rows is suggested.
However, in a case where a larger number of nozzles are provided in the head and a plurality of the heads are configured to be lined up in the sub-scanning direction to achieve higher speed, there is a problem in that there is a difference (for example, difference in concentration or the like) between an image formed on the regions of the upper and lower end portions of the recording medium in the sub-scanning direction (recording start portion and end portion) and an image formed on the center region, and the difference is visually recognized in a transition region thereof (a transition region which connects the regions of the upper and lower end portions to the region of the center portion).
Specifically, since the heads are configured to be lined up in the long region in the sub-scanning direction, there may be a case where partial overlap between the plurality of heads cannot be performed on the upper and lower end portions of the recording medium although partial overlap between scanning processes is possible. As a result, for example, in a case where there is a slight difference in characteristics between the heads (for example, the difference in the amount of ink droplets discharged from the nozzles), there may be a difference in image between the image region formed only by a single head corresponding to each of the upper and lower end portions and the image region subjected to POL control between the plurality of heads. In this case, there is a problem in that the difference between the images is visually recognized in the transition region.